This invention relates to a method of polishing a hard crystal substrate such as gallium nitride (GaN) and silicon carbide (SiC) substrates.
GaN (Group III nitride semiconductor) and crystalline material of SiC are wide band-gap semiconductors, and a GaN substrate is usually manufactured by forming a GaN film on a sapphire substrate by a halide vapor phase epitaxy (HVPE) method, as disclosed in Japanese Patent Publication Tokkai 9-335580. Devices may be formed on this GaN film, for example, to produce a short-wavelength green or blue light-emitting elements and a purple-color semiconductor laser. On the other hand, SiC is coming to be used as the substrate of a high-function power device since it can be used in high-output, high-frequency and high-temperature operations and has a high insulation breakdown field.
Since GaN substrates and SiC substrates are hard crystal substrates and since devices of specified kinds are formed on the surface of such a hard crystal substrate, high levels of flatness and smoothness are required of such surfaces.
Conventional methods of polishing the surface of such a hard crystal substrate were to use free polishing particles, initially carrying out a rough polishing process by using somewhat larger abrading particles and reducing stepwise the size of the abrading particles such that the substrate surface will become flat and smooth, as disclosed in Japanese Patent Publication Tokkai 2001-322899.
Polishing by free abrading particles is carried out by rotating a metallic lapping plate, supplying polishing slurry on the surface of this lapping plate, pressing the surface of a substrate held by a work holder onto the surface of the lapping plate and causing this work holder to rotate. In conventional polishing methods by free abrading particles, aqueous slurry with abrading particles dispersed in an aqueous dispersant has been used.
In conventional polishing methods by free abrading particles, furthermore, the technical problem has been to reduce the required polishing time, and the limit reachable by such methods has been to polish the target surface of a substrate to an average surface roughness (Ra) of about 0.4 nm. By such a conventional polishing method, however, unwanted scratch lines are formed, as shown in FIG. 16A, and it is presently not possible to sufficiently mirror-polish the surface of a hard crystal substrate.